In recent years, with higher integration and larger capacitance of large-scale integrated (LSI) circuits, the circuit line width required for semiconductor devices is becoming increasingly smaller. These semiconductor devices are manufactured by using an original pattern (also called a mask, photo mask, or reticle and hereinafter, referred to generically as a mask) to transfer the pattern onto a wafer by exposure in a reduced projection aligner, called a stepper, for circuit formation.
Improvement of die yields is indispensable to the manufacturing of LSI that requires a huge manufacturing cost. A leading cause that decreases yields is pattern defects of mask used for exposure and transfer of fine patterns onto a semiconductor wafer. As dimensions of LSI pattern formed on a semiconductor wafer become increasingly smaller in recent years, sizes of defects that need to be detected are becoming extremely small. Thus, a mask inspection apparatus that inspects for defects of masks used for the manufacturing of LSI needs to have high accuracy.
In the mask inspection apparatus, a mask surface is irradiated with an inspection light such as a coherent light and a reflected light or transmitted light thereof is collected by an image sensor to create images from collected data. At this point, the reflected light or transmitted light from the mask surface may be a very weak signal and thus, an image sensor with very high sensitivity is indispensable.
For an image sensor with very high sensitivity, environmental radiation such as cosmic rays and natural radiation could become noise sources. In such a case, it becomes impossible to distinguish between a signal caused by noise and that caused by defects of mask, affecting system performance significantly (See, for example, A. R. Smith et al., “Radiation events in astronomical CCD images”, SPIE 4669, 172-183 (2002)).
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